U.S. patent number 4,838,903 [Application Number 07/052,311] was granted by the patent office on 1989-06-13 for multi-phase thick-bed filter.
This patent grant is currently assigned to Ceco Filters, Inc.. Invention is credited to Steven I. Taub, Lazarus Thomaides.
United States Patent |
4,838,903 |
Thomaides , et al. |
June 13, 1989 |
Multi-phase thick-bed filter
Abstract
The invention being a multi-phase thick bed filter assembly
having a main filter element the primary function of which is to
remove aerosols from a moving gas stream and having an associated,
readily removable upstream pre-filter element designed to remove
solid particulate matter from the gas stream before its impingement
on the main filter.
Inventors: |
Thomaides; Lazarus (North
Wales, PA), Taub; Steven I. (Narbeth, PA) |
Assignee: |
Ceco Filters, Inc.
(Conshohocken, PA)
|
Family
ID: |
21976765 |
Appl.
No.: |
07/052,311 |
Filed: |
May 20, 1987 |
Current U.S.
Class: |
95/286; 55/480;
55/486; 55/498 |
Current CPC
Class: |
B01D
39/1623 (20130101); B01D 39/2017 (20130101); B01D
39/2082 (20130101); B01D 46/2411 (20130101) |
Current International
Class: |
B01D
39/16 (20060101); B01D 46/24 (20060101); B01D
39/20 (20060101); B01D 046/02 () |
Field of
Search: |
;55/97,480,481,482,486,498,DIG.25,508 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hart; Charles
Attorney, Agent or Firm: Seidel, Gonda, Lavorgna &
Monaco
Claims
We claim:
1. A fiber bed mist eliminator assembly for removal of aerosols,
including liquid and solid particulate matter, from a moving gas
stream, comprising: a main filter element secured in the assembly
containing a first fiber bed for removing aerosols from the gas
stream and a pre-filter element containing a second fiber bed
adjacent said main filter element and located upstream thereof,
said second filter bed adapted to remove solid particulate matter
from said gas stream before the gas stream contacts the main filter
element, and means for detachably retaining said pre-filter element
in the assembly, said retaining means further adapted to permit
easy removal of said pre-filter for cleaning or replacement without
removing the main filter element.
2. An assembly according to claim 1 wherein the second fiber bed is
sandwiched between support screens.
3. An assembly according to claim 1 wherein the second fiber bed
comprises fibers having a diameter from about 2 to about 100
microns and a packing density of from about 6 to about 12 pounds
per cubic foot.
4. An assembly according to claim 3 wherein the fibers have a
diameter from about 2 to about 100 microns.
5. An assembly according to claim 1 wherein the pre-filter element
is capable of removing solid particulate matter having a diameter
of 0.5 microns or greater from the moving gas stream.
6. An assembly according to claim 1 wherein the main filter is
supported by a tube sheet.
7. An assembly as claimed in claim 1 wherein the packing density of
the pre-filter element is substantially the same as the main filter
element.
8. A fiber bed mist eliminator assembly for removal of aerosols,
including liquid and solid particulate matter, from a moving gas
stream comprising: an upstanding main filter element supported by a
tube sheet and secured thereto containing a first fiber bed for
removing aerosols from the gas stream; an upstanding pre-filter
element comprising a second fiber bed sandwiched between a pair of
support screens for removing solid particulate matter from the gas
stream before the gas stream contacts the main filter element, said
pre-filter element disposed concentrically with said main filter
element and on the upstream side thereof; a top plate overlying the
open end of the assembly removably secured to the main filter
element; and means for detachably retaining said pre-filter element
in the assembly, said means further adapted to permit easy removal
of said pre-filter element for cleaning or replacement without
removing the main filter element.
9. An assembly according to claim 8 wherein the means for
detachably retaining said pre-filter in the assembly comprises a
member for securing the pre-filter element to the main filter
element.
10. An assembly according to claim 8 wherein a portion of said
second fiber bed extends beyond the ends of said support screens to
form a seal with said tube sheet or top plate.
11. A fiber bed mist eliminator assembly for removal of aerosols,
including liquids and solid particulate matter, from a moving gas
stream comprising: an upstanding main filter element supported by a
tube sheet and secured thereto containing a first fiber bed for
removing aerosols from the gas stream and an upstanding pre-filter
element comprising a second fiber bed sandwiched between support
screens, said pre-filter element adapted for removing solid
particulate matter from the gas stream before the gas stream
contacts the main filter element, said pre-filter element disposed
concentrically with said main filter element and on the upstream
side thereof; an opening formed in the tube sheet beneath the
filter assembly for the passage of the gas stream; a removable top
plate overlying said filter elements and having a concentrically
disposed aperture for receiving a rod means; a threaded aperture
supported in the opening of the tube sheet coaxially aligned with
the centrally disposed aperture in the top plate; said rod means
passing through said top plate aperture and threadably engaging the
apertures supported in the tube sheet opening, said top plate
secured to the filter assembly by a nut means on said rod.
12. An assembly according to claim 11 wherein the threaded aperture
supported in the opening of the tube sheet comprises a threaded nut
mounted on a cross bar diametrically spanning said tube sheet
opening.
13. A method for removal of aerosols, including liquids and solid
particulate matter from a moving gas stream comprising the steps
of: (a) providing a fiber bed mist eliminator assembly, comprising
a main filter element secured in the assembly, said main filter
element containing a first fiber bed for removing aerosols from the
gas stream, and (b) providing in said assembly a pre-filter element
containing a second fiber bed, (c) locating the prefilter element
adjacent the first fiber bed and upstream thereof, (d) removing
solid particulate matter from said gas stream before the gas stream
contacts the main filter element, (e) detachably retaining said
pre-filter element in the assembly to permit easy removal of said
pre-filter element for cleaning or replacement without removing the
main filter element, and (f) passing the gas stream through the
filter assembly to remove aerosols, including liquids and solid
particulate matter.
14. A method according to claim 13 wherein the second fiber bed
comprises fibers having a diameter from about 2 to about 100
microns and a packing density of from about 6 to about 12 pounds
per cubic foot.
15. A method according to claim 14 wherein the fibers have a
diameter from about 8 to about 50 microns.
16. A method according to claim 14 wherein the pre-filter element
removes solid particulate matter having a diameter of 3 microns or
greater from the moving gas stream.
Description
BACKGROUND OF THE INVENTION
Thick bed fiber filters, also called candles, aerosol filters, mist
filters and mist eliminators represent the most advanced method of
collection of fine particulates having a diameter of less than
three microns. This type of device has an efficiency in excess of
99% for particles having a diameter as small as 1/10th of a micron.
Processed gas contaminants, such as fumes, smog, oil and resin
smoke, are typically one micron or less in diameter.
These units are widely used in many industries to remove liquid and
semi-liquid mists carried in a process gas. In many cases, solid
particulates are also present as a result of the process
operations. Unfortunately, the candles are as efficient for solids
as they are for liquids. Because liquids drain continuously they do
not present a problem. Solids, however, tend to remain in the
filter, trapped between the interstices of the filter media.
Eventually, the filter becomes plugged rendering it ineffectual. As
the filter becomes impregnated with solids, the pressure drop
across the filter increases over a period of time but efficiency
remains constant. A filter change must be made to return the
pressure drop to a satisfactory level.
Prior art techniques for ameliorating this problem utilize "socks"
to protect the candles from plugging by accumulation of solid
particles. The socks comprise a layer of filter media having an
average packing density of only 2 pounds per cubic foot. The sock
is wrapped around the outside of the candle to trap particles
before they reach the aerosol primary filter. It was thought that
this would extend the useful life of the primary filter. This
concept did not work very well because of the low packing density
and large diameter fibers of the sock material. Other methods of
pressure drop recovery have included washing in place or washing
externally using the appropiate washing medium to dissolve the
solids and semi-solids from the filter media or by installing a new
set of filters. Washing is expensive and time consuming. Moreover,
the degree of recovery achieved using such a procedure is affected
by the type and size of collected solids and the rapidity of solid
dissolution by the washing media. Experience has shown that only a
portion of the pressure drop is recovered and the effect of the
wash cycle will eventually require main filter replacement.
SUMMARY OF THE INVENTION
A thick-bed filter assembly for removal of aerosols and solid
particulate matter from a moving gas stream is provided. A main
filter element in the assembly contains a first fiber bed for
removing aerosols from the gas stream. The assembly also contains a
pre-filter element containing a second fiber bed adjacent to the
main filter element and located upstream thereof. The pre-filter
removes solid and liquid particulate matter from the gas stream.
The assembly contains means for detachably retaining the pre-filter
element in the assembly to permit its easy removal for cleaning or
replacement without removing the main filter element.
According to one embodiment of the invention, an upstanding main
filter element is supported by a tube sheet. An upstanding
pre-filter element is disposed concentrically of the main filter
element and on the upstream side thereof. A top plate overlying the
open ends of the assembly is removably secured to the main filter
element.
In yet another embodiment the main filter element is supported by a
tube sheet. The tube sheet contains an opening beneath the filter
assembly for the passage of gas. A removable top plate overlying
the filter elements has a centrally disposed aperture for receiving
a rod means. A threaded aperture is supported in the opening of the
tube sheet coaxially aligned with the centrally disposed aperture
in the top plate. A rod means passing through the top plate
aperture threadably engages the threaded aperture supported in the
tube sheet opening. The top plate is secured to the filter assembly
by a nut means on the rod.
A method for removal of aerosols and solid particulate matter from
a moving gas stream is also provided. A gas stream is passed
through a thick-bed filter assembly having a main filter element
containing a first fiber bed for removing aerosols from the gas
stream. The filter assembly has a pre-filter element containing a
second fiber bed adjacent to the main filter element and located
upstream thereof for recovering solid particulate matter from the
gas stream. The assembly contains means for detachably retaining
the pre-filter element in the assembly to permit easy removal of
the pre-filter for cleaning or replacement without removing the
main filter element.
It has been discovered that the initial 1/4 inch to 1/2 inch of the
filter media does the bulk of the trapping and holding of the
contaminating material, including solids.
Accordingly, it is an object of the present invention to provide a
multi-phase filter assembly in which that portion of the filter
initially contacted by the gas stream can easily be removed and
replaced.
It is further object of this invention to provide a multi-phase
filtering system in which the pre-filter is designed to remove
solid particulate matter to such a degree that its replacement
returns the system to substantially pristine condition.
It is a still further object of the invention to provide a
thick-bed filter assembly meeting the above criteria which reduces
maintenance time, and which has little or no effect on the
up-stream process conditions with which it is associated.
It is yet another object of the invention to provide a
multiple-phase filter assembly designed and constructed to reduce
the cost of filter replacement and change-over time, including
minimal down-time for the process system itself. In achievement of
this end, the initial layers of the filter are structured as
sub-assemblies with the total composite designed to meet the
requirements of liquid and solid collection. More particularly, the
pre-filter is sandwiched between cages so that replacement time is
reduced to a minimum.
For the purpose of illustrating the invention, there is shown in
the drawings forms which are presently preferred; it being
understood, however, that this invention is not limited to the
precise arrangements and instrumentalities shown.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the filter elements of a thick-bed
filter assembly designed for an inside/out gas flow system in which
the pre-filter is in the form of a caged insert.
FIG. 2 is a perspective view of the filter elements of an
outside/in gas flow system using a caged pre-filter positioned on
the outside of the main filter.
FIG. 3 is a cross-sectional view of an inside/out thick-bed filter
assembly using a replaceable pre-filter constructed in accordance
with the present invention.
FIG. 4 is a sectional view of the composite filter taken in the
direction of the arrows along the section line 4--4 of FIG. 3.
FIG. 5 is a cross-sectional view of an outside/in thick-bed filter
assembly constructed in accordance with the present invention.
FIG. 6 is a sectional view of the composite filter taken in the
direction of the arrows along the section line 6--6 of FIG. 5.
FIG. 7 is a diagramatic representation showing the various modes of
mounting the thick-bed filter assembly to a tube sheet.
FIG. 8 is another thick-bed filter assembly accordingly to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Depending on the direction of gas flow, the pre-filter can either
be placed around the outside of the main filter or can be inserted
internally into it. The prior art "sock" was not capable of the
latter since it was not rigid. It was merely wrapped around the
outside of the main filter, and depended on the main filter for
structural support. The sock could not be used in inside/out type
filtering operations.
The pre-filter bed media is generally selected from an inexpensive
material in comparison with the main filter material. Since the
pre-filter has a much shorter intended life, less expensive
materials are acceptable. Thus, whereas chemical grade fiberglass
may be used for the main filter, the pre-filter could be special
grade fiberglass or polyester. The pre-filter is most
advantageously formed of a bed of fibers having a fiber diameter
from about 2 to about 100 microns, with about 8 to about 50 microns
being preferred.
Another parameter governing the effectiveness of the pre-filter
media is packing density. Typically, packing densities of about 6
to about 12 pounds per cubic foot are used. While higher packing
densities may be possible, the pressure drop would be adversely
increased. If packing densities of less than 6 pounds per cubic
foot are employed, the pre-filter would not be efficient enough and
will allow solid particles to pass through and impinge on the main
filter.
The preferred thickness of the pre-filter bed is between 1/4 and
3/4 inches, but other thicknesses may be employed. The pre-filter
thickness is largely dependent on the specific application.
The pre-filter is capable of removing solid particulate matter from
the gas stream having a diameter of 0.5 microns or greater.
As previously noted, the pre-filter may be formed of any material
suitable for the service required, such materials include
polyesters, polypropylene, special grade fiberglass, chemical grade
fiberglass, ceramic fibers, particularly aluminum silica fibers,
e.g., mullite, and flurocarbon materials such as "TEFLON", made
into fibers.
The principal advantage of the invention is that the pre-filters
are field changable, that is, the operator can remove and replace
the pre-filter without disassembling the filter apparatus or
removing the main filter. The normal factors which limit the life
of aerosol filters are corrosion of the cages and filter material,
and more importantly, plugging of the filter material by solids. In
instances where the pre-filter is 100% successful in keeping solids
off of the main filter and the gas is non-corrosive, the life of
the main filter can be extended almost indefinitely. By providing
for ready removal of the pre-filter, operation of the underlying
process may be continued substantially without interruption.
With particular reference to FIGS. 1 and 2 of the drawings, there
is illustrated the two basic forms of prefilters employed in the
invention. FIG. 1 depicts the form of the invention in which the
pre-filter 12 is inserted into cavity 14 formed by the main aerosol
filter 10. FIG. 2 depicts the form of the invention in which the
pre-filter 12 is positioned externally of the main filter 10. When
the flow of gas is in the direction shown by arrows in FIG. 1, the
solid contaminants accumulate on the inside of the candle. In FIG.
2 with gas flow as shown by the arrows, the solid contaminants will
accumulate on the outside of the candle.
The filters need not necessarily be cylindrical candle filters as
shown in FIGS. 1 and 2, but may have any desired polygonal
cross-section in which filter layers are provided in any desired
manner supported by suitable framework.
The main filter media is typically comprised of a bed 16 of
randomly distributed fibers having a mean fiber diameter of at
least about 2 to 50 microns packed to a density of from 6 to 12
pounds per cubic foot. Suitable fiber materials, may include, for
example, fibers of polymeric material such as polyesters,
polyvinylchloride, polyethelene, flurocarbons, nylons,
polypropylene; glass fibers; and ceramic fibers.
Preferably, the pre-filter media forms a bed 20 sandwiched between
support screens 18 of an open network material for structural
support. These screens, sometime referred to as "cages" may be
formed from plastic, metal or other suitable material for
structural support. When the pre-filter becomes loaded with solid
particles, it can easily be removed from the filter assembly and
replaced. As will hereinafter be noted, the pre-filter is always
installed on the pressure side of the main or candle filter, or
what is hereinafter described as the upstream side of the
assembly.
The main filter media 16, like the pre-filter media, may form a bed
sandwiched between cages 17 of a plastic, metal or other suitable
material for structural support. Similarly, in FIG. 2 main filter
media 16 may form a sandwich bed between cages 18.
In a preferred embodiment, the pre-filter bed 20 extends 1/2 inch
to 1 inch past the ends of the cage to form an overhang as shown in
FIGS. 1 and 2. As will be seen in FIGS. 3 and 5, this overhang
allows the pre-filter to act as a seal preventing gas from reaching
the main filter prior to pre-filtering by the pre-filter.
Referring to FIG. 3, two multi-phase filter assemblies generally
designated 19 are supported side-by-side on tube sheet 22. For
purposes of illustration, only two such assemblies are shown, it
being understood that any number of assemblies may be contained in
the treatment vessel 26. To permit ready access to the filter
assemblies, access openings 47 may be provided at the top of the
vessel. The assemblies are of the inside/out flow type, that is,
processed gas enters from an inlet (not shown) in the bottom of the
vessel and travels upward through openings 27 in the tube sheet and
into the annular space defined by pre-filter 12. The tube sheet
spans the inside of vessel 26, separating it into high and low
pressure zones 34 and 36, respectively. The gas then flows radially
outward through the pre-filter 12 and filter 10. Solids are
collected by the pre-filter while aerosol-laden gas is further
filtered by the main filter 10. The thus-filtered processed gas,
substantially free of solid and liquid particulates, exits the
vessel from port 38.
An annular ring 40 is affixed, e.g., by welding or other means to
the outer periphery of cage 17 of the main filter, adjacent to the
top end of the filter. A series of radially outwardly extending
plates 42 are spaced equidistant around ring 40 to form a type of
external spider around the filter assembly. The top of each filter
assembly is closed by a top plate 23. The plate is secured in place
by bolts 44 which pass through orifices in the tube plate and
mating orifices 43 in each of plates 42. The bolts 44 are secured
by nuts 46. Referring to FIG. 4, it can be readily appreciated that
top plate 23 must contain orifices intersperced around its
periphery and in alignment with the orifices 43 on plate 42 to
accommodate bolts 44. The bolts 44 effectively clamp the top plate
to the filter assembly, thereby creating a seal.
The embodiment of FIG. 3 is one manner of adapting the present
invention to an existing aerosol filter installation which utilizes
inside/out candle filters supported by a tube sheet.
The portion of the pre-filter bed 20 which extends and overhangs
beyond the pre-filter cages 18 acts as a gas seal. Top plate 23
pressing down on the filter assembly causes compression of the
pre-filter bed 20 into a generally mushroom shape 25 thereby
sealing off the top end of the main filter and preventing gas from
reaching the main filter before passing through the pre-filter.
It may be noted in FIG. 3 that the pre-filter 12, unlike the main
filter 10, is not always supported by the tube sheet 22. It may
therefore be necessary to attach the prefilter to the main filter
to prevent the former from falling out of the assembly.
Means for detachably retaining the pre-filter to the main filter
are provided. The means may take the form of any suitable temporary
attachment device such as clips, clamps, wire, twist ties, etc.
The pre-filter 12 is most advantageously connected to the inner
cage 17 of the main filter 10 by a wire tie 28. The tie can be
simply run through the cages and tied off.
While the filter assembly is shown in FIG. 3, for purposes of
illustration, supported by a tube sheet, it should be understood
that, as shown in FIG. 7, the filter assemblies may, alternatively,
be suspended from the tube sheet.
Referring to FIG. 5, two multi-phase filter assemblies generally
designated as 49 are supported side-by-side on tube sheet 22. The
main and pre-filter elements are constructed as in FIGS. 1-3, that
is, each member is formed from a fiber bed sandwiched by suitable
supporting cages. For ease of illustration, the containment vessel
is omitted, it being understood that the tube sheet 22 otherwise
separates a vessel into high and low pressure zones as in FIG. 3,
it being further understood that the zones are reversed in FIG. 5.
The assemblies are of the outside/in flow type, that is, gas flows
radially inward from the high pressure zone of the vessel, through
the filter beds and into the cylindrical space within the main
filter 10 and out the open end of the assembly, which is supported
by tube sheet 22.
FIG. 5 represents an alternate embodiment for mounting the
pre-filter element within the assembly using an internal spider. A
spider 48 is mounted to a band 50 which is secured, as by welding,
to the inner cage 17 of the main filter element 10. The spider has
a central boss 54 provided with a threaded hole 55 adapted for
engagement by a threaded bolt 56, as seen in FIG. 5. The bolt
extends upward into a hole provided in the top cover plate 23. A
compression nut 60 secures the top plate 23 in sealing engagement
with the pre-filter bed 20 compressing it into a mushroom shape
25.
The pre-filter 12 is supported on the tube sheet 22 as is the main
filter element 10. In this form of assembly the pre-filter element
is self-supporting on tube sheet 22. It can readily be appreciated
that the means for detachably retaining the pre-filter in the
assembly in this embodiment of the invention comprises the tube
sheet itself in supporting the pre-filter against the force of
gravity. In the embodiment of FIG. 5, the pre-filter is also held
in place by top plate 23 which is removably secured across the top
of the filter assembly.
FIGS. 3 and 5 depict different structural designs for holding the
top plate 23 securely in place while at the same time permitting
easy removal of the pre-filter from the assembly when its
replacement is required. In both embodiments, the pre-filtering
element 12 can be readily removed from the system by the simple
expedient of removing the compression nuts, and in the case of FIG.
3, additionally cutting the ties 28. Securement of the main filter
element to the tube sheet is by means well known in the art.
Where the prefilter is positioned external of the main filter
element (as shown in FIG. 5), the diameter of top plate 23 may, in
some instances, be less than the inside diameter of the pre-filter
12. In such cases (not shown) the prefilter may be lifted from the
assembly without removing the top plate.
By placement of the pre-filter in the manner shown in FIGS. 3 and
5, all of the gas passes through the pre-filter before impinging on
the main filter. Efficiency of the pre-filter is effectively about
90%, this being a measure of the liquids and solids which it
filters out.
The filter assembly may be installed in the vessel either suspended
from the tube sheet or mounted to the tube sheet in upright
position. FIG. 7 shows, in diagramatic form, the various
arrangements for mounting the filter assembly to the tube sheet.
The direction of gas flow in each instance is indicated by the
arrows. Viewing the arrangements from left to right there is shown
in A a filter assembly in which the pre-filter element 12 is
disposed internally of the main filter element 10. The main filter
is mounted in place by plates 62 secured to the filter and bolted
to the tube sheet 22. In the unit marked B the pre-filter 12 is
mounted externally of the main filter 10. The pre-filter is held in
place by being tied to the main filter cage 17 by tie 28.
Assemblies A and B in FIG. 7 hang from the tube sheet 22.
Assemblies C and D are mounted on top of the tube sheet and show,
respectively, the pre-filter 12 mounted internally and externally
with respect to the main filter 10. While the pre-filter is
supported by tube sheet 22 in D, it must be fastened to the main
filter in C.
Yet another embodiment of the multi-phase filter assembly is shown
in FIG. 8. Gas flow is in the direction indicated by the arrows. A
cross bar 70 is fixed, for example by welding, across the opening
in tube sheet 22. An annular band 72 is similarly fixed, such as by
welding, to the inner periphery of the inner cage 17 of the main
filter element 10 adjacent the lower extremity of the main filter
element. Band 72 is used to properly seat the filter within the
tube sheet opening when the filter is installed from a top access
opening. A second cross bar 76 spans the inside diameter of annular
band 72 and is fixed thereto, such as by welding. Hex nuts 78 are
attached at a central location to each of the cross bars 70 and 76.
A rod 80 is threaded through the hex nuts 78 and secures a top
plate 82 in place over the assembly by means of a compression nut
84. The main filter element 10 is prevented from being contacted by
the gas stream until it passes through the pre-filter 12 by means
of the seal formed between the top plate 82 and pre-filtering bed
20 in the manner previously noted. To perfect the seal a gasket 86
is interposed between the candle flange 88 and tube sheet 22.
Tightening nut 84 compresses both the gasket 86, the filter bed 20,
and filter element 10.
While for purposes of illustration the pre-filter is shown located
internally of the main filter in FIG. 8, it should be appreciated
that the pre-filter and filter may be reversed, depending on the
application.
In each of the embodiments shown in FIGS. 3 through 8, means are
provided for detachably securing the pre-filter element within the
filtering assembly. By the simple expedient of loosening one or
more compression nuts, and/or severing a restraining tie, and/or
simply lifting the pre-filter from the assembly, the pre-filter can
be quickly and easily removed from the assembly for replacement
without removing the main filter.
The present invention may be embodied in other specific forms
without departing from the spirit or essential attributes thereof
and, accordingly, reference should be made to the appended claims,
rather than to the foregoing specification, as indicating the scope
of the invention.
* * * * *